NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

Support for this project was provided by Contract NASW 01001 between the National Academy of Sciences and the National Aeronautics and Space Administration. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsor.

International Standard Book Number-13: 978-0-309-10484-5

International Standard Book Number-10: 0-309-10484-X

Cover: Cover design by Penny E. Margolskee. The lower half of the cover is an image of a microbial mat community found at a depth of about 15 meters in Lake Vanda, Wright Valley, Antarctica; photo courtesy of Dale Andersen, SETI Institution. The upper half of the cover is a composite image of the Orion Nebula made by combining data from the Hubble Space Telescope and the Spitzer Space Telescope; image courtesy of NASA/Jet Propulsion Laboratory-California Institute of Technology, T. Megeath (University of Toledo), and M. Robberto (Space Telescope Science Institute). The crescent on the center right is an ultraviolet image of the planet Venus as seen by the Hubble Space Telescope; image courtesy of NASA/JPL/Space Telescope Science Institute.

A limited number of copies of this report are available free of charge from:

THE NATIONAL ACADEMIES

Advisers to the Nation on Science, Engineering, and Medicine

The National Academy of Sciences is a private, nonprofit, self-perpetuating society of distinguished scholars engaged in scientific and engineering research, dedicated to the furtherance of science and technology and to their use for the general welfare. Upon the authority of the charter granted to it by the Congress in 1863, the Academy has a mandate that requires it to advise the federal government on scientific and technical matters. Dr. Ralph J. Cicerone is president of the National Academy of Sciences.

The National Academy of Engineering was established in 1964, under the charter of the National Academy of Sciences, as a parallel organization of outstanding engineers. It is autonomous in its administration and in the selection of its members, sharing with the National Academy of Sciences the responsibility for advising the federal government. The National Academy of Engineering also sponsors engineering programs aimed at meeting national needs, encourages education and research, and recognizes the superior achievements of engineers. Dr. Charles M. Vest is president of the National Academy of Engineering.

The Institute of Medicine was established in 1970 by the National Academy of Sciences to secure the services of eminent members of appropriate professions in the examination of policy matters pertaining to the health of the public. The Institute acts under the responsibility given to the National Academy of Sciences by its congressional charter to be an adviser to the federal government and, upon its own initiative, to identify issues of medical care, research, and education. Dr. Harvey V. Fineberg is president of the Institute of Medicine.

The National Research Council was organized by the National Academy of Sciences in 1916 to associate the broad community of science and technology with the Academy’s purposes of furthering knowledge and advising the federal government. Functioning in accordance with general policies determined by the Academy, the Council has become the principal operating agency of both the National Academy of Sciences and the National Academy of Engineering in providing services to the government, the public, and the scientific and engineering communities. The Council is administered jointly by both Academies and the Institute of Medicine. Dr. Ralph J. Cicerone and Dr. Charles M. Vest are chair and vice chair, respectively, of the National Research Council.

Preface

As the search for life in the solar system expands, it is important to know what exactly to search for. Previous life-detection experiments have been criticized for being too geocentric. This study aims to inform research program managers, policymakers, and mission designers about the possibilities for life on other solar system bodies. Further, during planetary protection exercises at the National Aeronautics and Space Administration (NASA), questions concerning the possibility of nonterrana life recur repeatedly. Remarkably little knowledge is organized that might shed light on the plausibility of bizarre life as a concern for planetary protection.

The search for signs of life, present or past, is an important goal of NASA’s robotic solar system exploration programs and, ultimately, for its astronomical programs designed to probe the gross characteristics of extrasolar planetary systems. To date, that search has been governed by a model of life that is based on the life that we know on Earth—terran life. Several features of terran life have attracted particular focus:

Terran life uses water as a solvent;

It is built from cells and exploits a metabolism that focuses on the carbonyl group (C=O);

It is thermodynamically dissipative, exploiting chemical-energy gradients; and

It exploits a two-biopolymer architecture that uses nucleic acids to perform most genetic functions and proteins to perform most catalytic functions.

As a consequence, most of NASA’s mission planning is focused on locations where liquid water is possible, and it emphasizes searches for structures that resemble cells of terran organisms, small molecules that might be the products of carbonyl metabolism, particular kinds of chemical-energy gradients, and tests for amino acids and nucleotides similar to those found in terrestrial proteins and DNA. This approach is defensible given the absence of a general understanding of how life might appear if it had an origin independent of Earth. Experiments in the laboratory, however, are suggesting that life might be based on molecular structures substantially different from

a

The Committee on the Limits of Organic Life in Planetary Systems uses the term “terran” to denote a particular set of biological and chemical characteristics that are displayed by all life on Earth. Thus “Earth life” has the same meaning as “terran life” when the committee is discussing life on Earth, but if life were discovered on Mars or any other nonterrestrial body, it might be found to be terran or nonterran, depending on its characteristics.

those known in contemporary terran life. These results suggest that if life originated independently, even within our own solar system, it might have nonterran characteristics and, thus, not be detectable by NASA’s in situ or remote-sensing missions designed explicitly to detect terran biomolecules or their products.

Further, if life is possible in solvents other than liquid water, it might exist in planetary environments other than the few that are currently targeted as potential hosts of nonterran life. Other than on Earth, liquid water is now considered possible only on subsurface Mars and in sub-ice environments of the Galilean moons of Jupiter (Europa, Ganymede, and Callisto), and perhaps on Saturn’s moon, Enceladus. Nonaqueous solvents might, however, be present in other planetary environments. Because some of these spots (e.g., the surface of Titan) could be more accessible via spacecraft missions than either the deep subsurface of Mars or sub-ice Europa, evidence for life in solvents other than water might redirect missions to these other locales, and substantially improve the design of life-detection instrumentation generally. Similarly, nonterran life may change the gross characteristics of planetary environments in ways that differ from influences stemming from terran life, and these differences (e.g., the relative abundances of atmospheric species) may ultimately be observable over interstellar distances with astronomical facilities now on the drawing board.

This report explores a limited set of hypothetical alternative chemistries of life by following a hierarchy of possibilities that have been ranked through experimental, exploratory, and theoretical work done in the past. The study briefly reviews current knowledge concerning the following questions or hypotheses and provides suggestions for future research.

What environments on Earth that are extremes by terran standards harbor life? How must life-detection strategies be altered to discover this life on Earth? What extreme environments have not received attention? Are there synthetic environments that better represent conditions on alien worlds?

What environments on Earth are so extreme that life with standard terran biochemistry has been unable to occupy it?

What life forms are possible, still based on carbon and still functioning in water, but with a fundamental difference in the method of reproduction? Issues to be explored include the following:

What types of polymeric structures, other than proteins built from the standard 20 amino acids, might support catalysis in water? For example, can 2-amino-2-methyl-carboxylic acids, which have been found to be enantiomerically enriched in meteorites, be the basis for a catalytic system? In the absence of biopolymers, would selected monomers provide catalysis sufficient to sustain life?

What types of polymeric structures, other than nucleic acids built from the standard four nucleotides, might be replicatable and might support Darwinian evolution in water?

Can a functioning genetic system be established that is not based on a linear molecular structure? For example, can a compositional genome (a collection of monomers) sustain heredity?

Can a system capable of Darwinian evolution be demonstrated in the laboratory using nonstandard biopolymers or a compositional genome in water?

What life forms are possible, still based on carbon, but not functioning in water? Issues to be explored include these:

Can membranes be constructed in the laboratory that separate an organic solvent inside a cell from an organic solvent outside a cell?

What kinds of polymeric structures (or monomer collections) might support catalysis and genetics in nonaqueous environments, particularly in solvents found on solar system bodies other than Earth?

Can mineral systems be identified that interact in interesting ways with organic compounds in nonaqueous systems?

Can asymmetric induction, and spontaneous resolution that leads to the homochirality assumed to be necessary for life, be achieved in nonaqueous solvents, especially those found on solar system bodies other than Earth?

Can a system capable of Darwinian evolution be demonstrated in the laboratory using nonstandard monomers and/or biopolymers in nonaqueous environments?

To evaluate the possibility that nonstandard biochemistry (i.e., biochemistry different from what we find as the universal biochemistry on Earth) might support life in known solar system environments and conceivable extrasolar environments; and

To define broad areas that might guide NASA and the National Science Foundation to fund efforts to expand knowledge in this area.

The results of this study are meant to aid in the development of a new generation of life-detection experiments that can be conducted in situ on planetary surfaces or conducted on samples returned from other solar system bodies.

Held on April 25, 2002, at the National Academies’ Georgetown facility in Washington, D.C., the “weird life” planning session was chaired by John Baross (University of Washington) and included presentations from Chris Chyba (SETI Institute and Stanford University), Steven Benner (Foundation for Applied Molecular Evolution), Jack Szostak (Harvard University), George Cody (Carnegie Institution of Washington), and Robert Shapiro (New York University). A discussion session was led by Mitch Sogin (Woods Hole Marine Biological Laboratory). A planning session for the Workshop on the Limits of Organic Life in Planetary Systems was held at the Constitution Avenue building of the National Academies in Washington, D.C., on March 2-3, 2004, and chaired by John Baross with input from NASA staff members Michael Meyer, Marc Allen, and John Rummel.

A writing meeting was held on March 14-16, 2005, at the National Academies’ Arnold and Mabel Beckman Center, Irvine, California, and chaired by John Baross (University of Washington), with presentations from Steven Benner (Foundation for Applied Molecular Evolution), William Baines (Rufus Scientific), and Jonathan Lunine (University of Arizona).

Acknowledgments

The committee thanks Space Studies Board (SSB) interns Stephanie Bednarek, Matthew Broughton, and Brendan McFarland and Board on Life Sciences program officer Evonne Tang for their work on compiling the glossary and researching references. The committee also thanks SSB research assistant Victoria Swisher for assistance with the report review process.

This report has been reviewed by individuals chosen for their diverse perspectives and technical expertise, in accordance with procedures approved by the National Research Council’s (NRC’s) Report Review Committee. The purpose of this independent review is to provide candid and critical comments that will assist the authors and the NRC in making the published report as sound as possible and to ensure that the report meets institutional standards for objectivity, evidence, and responsiveness to the study charge. The contents of the review comments and draft manuscript remain confidential to protect the integrity of the deliberative process. We wish to thank the following individuals for their participation in the review of this report:

Robert H. Austin, Princeton University,

Paul Davies, Macquarie University, Australia,

Jack Farmer, Arizona State University,

Katherine H. Freeman, Pennsylvania State University,

James F. Kasting, Pennsylvania State University,

Anthony Keefe, Archemix Corporation,

Peter B. Moore, Yale University,

Kenneth H. Nealson, University of Southern California, and

Norman H. Sleep, Stanford University.

Although the reviewers listed above have provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations, nor did they see the final draft of the report before its release. The review of this report was overseen by Leslie Orgel, Salk Institute for Biological Studies. Appointed by the National Research Council, he was responsible for making certain that an independent examination of this report was carried out in accordance with institutional procedures and that all review comments were carefully considered. Responsibility for the final content of this report rests entirely with the authoring committee and the institution.

The search for life in the solar system and beyond has to date been governed by a model based on what we know about life on Earth (terran life). Most of NASA's mission planning is focused on locations where liquid water is possible and emphasizes searches for structures that resemble cells in terran organisms. It is possible, however, that life exists that is based on chemical reactions that do not involve carbon compounds, that occurs in solvents other than water, or that involves oxidation-reduction reactions without oxygen gas. To assist NASA incorporate this possibility in its efforts to search for life, the NRC was asked to carry out a study to evaluate whether nonstandard biochemistry might support life in solar system and conceivable extrasolar environments, and to define areas to guide research in this area. This book presents an exploration of a limited set of hypothetical chemistries of life, a review of current knowledge concerning key questions or hypotheses about nonterran life, and suggestions for future research.

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